There are many methods and apparatus disclosed in the prior art for removing minerals from water. A water softening apparatus such as that disclosed in U.S. Pat. No. 3,891,552 is an example of a water treatment system that is used to remove certain “hard” ions in order to produce softened water. When substantially all of the ion sites in the water softening resin hold a “hard” ion, the resin must be regenerated. In a typical water softener of the “ion exchange” type, a resin tank containing a water softening resin is utilized. In particular, the “hard” water is passed through the resin tank where the water exchanges its “hard” ions of calcium, magnesium, etc. for “soft” sodium or potassium ions located at the resin ion exchange sites. The resin is selected to have a greater affinity for the calcium and magnesium ions and thus releases the sodium or potassium ions in favor of the calcium and magnesium ions carried by the water.
The AWWA in their Handbook, Water Quality and Treatment (published by the American Water Works Association, fourth edition, 1990, pages 657 and 658), says “Hardness in natural water is caused by the presence of any polyvalent metallic cation . . . . Because the most prevalent of these species are the divalent cations of calcium and magnesium, total hardness is typically defined as the sum of the concentrations of these two elements and is usually expressed in terms of milligrams per liter as CaCO3.”
Table 10.4 on page 658 of this reference lists the following.
Hardness Range(mg/L as CaCO3)Hardness Description0-75Soft75-150Moderately Hard150-300 Hard>300Very Hard
In a conventional water softener, a brine solution is flushed through the resin bed to regenerate the resin. The high concentration of sodium or potassium ions in the brine solution forces the resin bed to release the calcium and magnesium ions which are discharged to a drain. At the end of the regeneration cycle, the ion exchange sites in the resin bed each hold sodium or potassium ions. The regeneration cycle typically lasts about an hour and needs to be done several times a week. More frequent regenerations may be required in periods of greater than normal water usage. As should be apparent, a water softener of the type described produces a waste stream during regeneration that contains brine. In some locations of the country, the discharge of a brine solution from a water treatment system is restricted or may be prohibited in the future.
Deionization or demineralization systems are also available in the prior art for removing both cations and anions from a water supply. An example of such a deionization system can be found in U.S. Pat. No. 4,427,549. In the disclosed system, separate cation and anion resin tanks are used to remove cations and anions, respectively from the water being treated. The cation and anion tanks contain respective cation and anion exchange resins.
Like the resin tank described above in connection with the water softening apparatus, the resin tanks of the deionization apparatus must be regenerated periodically to flush the captured ions from the resins. In a deionization apparatus of the type disclosed in the '549 patent, the cation resin is regenerated by an acid regeneration solution which drives the cations from the resin bed and replaces them with hydrogen ions (H^+). The anion resin is regenerated by an alkaline solution which flushes the anions from the resin bed and replaces them with hydroxyl ions (OH^−). In this type of deionization apparatus, two waste streams are produced during regeneration, one being an acid solution, the other being an alkaline solution. A water deionization system where waste streams of this type are eliminated or substantially reduced is desirable.
This desired result has been previously accomplished by using an electrodeionization (EDI) apparatus. Conventional EDI water producing methods as described in U.S. Pat. No. 7,033,472 contain an ion depletion chamber partitioned by a cation exchange membrane on one side and anion exchange membrane on the other side. The depletion chamber is packed with an ion exchange material. Concentrate chambers are provided on both sides of the depletion chamber with the cation exchange membrane and anion exchange membrane in between. The depletion chamber and the concentration chambers are disposed between an anode chamber having an anode and a cathode chamber having a cathode. In many instances, the depletion and concentrating chambers are stacked in multiples to achieve the desired flow rate by having cation exchange membranes and anion exchange membranes, separated from one another, alternately arranged and an ion exchange material filling every other chamber formed by the cation exchange membrane and anion exchange membrane. Ion exchange material may also be in the concentrate chambers as well. Water to be processed is supplied to the depletion chamber while applying a voltage. Concentrate water is sent to a concentrate chamber to remove impurity ions from the water to be processed, whereby deionized water is produced. Another example of an EDI system is disclosed in U.S. Pat. No. 4,871,431.
An EDI apparatus as described in U.S. Pat. No. 6,607,647 describes EDI as a process that removes ionizable species from liquids using electrically active media and an electrical potential to influence ion transport. In EDI the ability of the resin to rapidly transport ions to the surface of the ion exchange membranes is much more important than the ion exchange capacity of the resin. Therefore resins are not optimized for capacity but for other properties that influence transport, such as water retention and selectivity.
Many other commercial EDI systems currently have limitations that prevent the use on typical well and public water sources. Such limitations may include the requirement for softening or reverse osmosis pretreatment methods to prevent scaling that would lead to a device failure, inability to process intermittent flows, and very limited ion exchange reserve capacity. If there is no product water flow then the device cannot regenerate itself as designed. Other similar devices are designed for high purity water production and cannot treat water with high concentrations of ions, making them impractical for normal household use. Therefore a device that could overcome these limitations is desired.